The present invention relates to a process for preparing a powder of uranium dioxide with physico-chemical properties suitable for the preparation of a uranium and plutonium mixed oxide (MOX) fuel used in light water reactors.
More precisely, it relates to obtaining a powder which is castable, fine and with restricted particle size, intended to be mixed with another powder (a primary mixture rich in Pu) and which has the following properties:
good spontaneous flow properties,
a homogeneous particle size located in the range between 20 and 100 xcexcm,
a high apparent density to allow optimum filling of different equipment used for manufacturing fuel (grinder, mixer, containers, transfers, pressure foot feed, etc.),
elementary particles sufficiently solid to withstand the different mechanical stresses undergone during manufacture (convection mixing, filling and emptying containers, transfer by pneumatic track, etc.),
good compaction capability, and
excellent reactivity to natural sintering.
At present, a powder of uranium dioxide with these properties is obtained by preparing it by a wet conversion process of uranyl nitrate in uranium dioxide.
This wet conversion process consists of a precipitation of ammonium diuranate (ADU) followed or not by grinding to the appropriate particle size, as described in U.S. Pat. No. 3,394,997 [1], and U.S. Pat. No. 3,998,925 [2]. Document FR-A-2 088 170 [3] describes a conversion process of uranyl nitrate in sinterable uranium dioxide by drying-pulverisation of a solution of uranyl nitrate and formic acid, followed by calcination.
The powders obtained by these wet conversion processes can be used directly for the industrial manufacturing of MOX fuel without it being necessary to carry out a supplementary stage of mechanical granulation of the powder.
In fact, the powders have excellent flow properties, which give the final UO2xe2x80x94PuO2 mixture sufficient fluidity for withstanding the high output rates.
On the other hand, the preparation process of uranium dioxide using dry conversion of uranium hexafluoride UF6 in UO2, such as described in FR-A-2 060 242 [4] and FR-A-2 064 351 [5], does not make it possible at present to obtain a powder able to be used directly for manufacturing MOX fuels. Dry conversion of UF6 is carried out industrially in a compact oven and plays on two reactions consecutively:
the first is the hydrolysis reaction which transforms gaseous UF6 into solid uranium oxyfluoride UO2F2,
the second relates to the reducing pyrohydrolysis of UO2F2, which leads to the formation of UO2 under pulverulent form.
The UO2 powder produced according to the process described in [4] and [5] is difficult to use for preparing MOX fuels since it is usually cohesive and has a low apparent density, which makes direct utilisation very tricky for industrial application because of its very poor compaction behaviour.
When this process according to references [4] and [5] is used to prepare fuels with a base of UO2 for reactors, granulation of the powder is next carried out by a mechanical process comprising a pre-compaction of the powder, followed by a crushing, then a sieving in order to obtain a mechanical granulate with good flow properties, which in certain circumstances is submitted to a spheroidizing operation to increase the castability of the powder further. This mechanical granulation leads to high particle sizes which can reach and even exceed 500 xcexcm.
Another powder granulation technique for UO2 has been described in FR-A-1 438 020 [6]. In this case, a paste is prepared by mixing a powder of UO2 or UO2xe2x80x94PuO2, with a solution of a binder in an organic solvent with a very low hydrogen content, such as trichloroethylene, then drying this paste by pulverisation. Thus granulates of high dimension reaching 250 xcexcm are obtained.
For manufacturing MOX fuel, where an intimate mixture of the two constituents (UO2 powder and PuO2 powder) is needed, such high particle sizes cannot be allowed for UO2. It is indispensable to have UO2 particle sizes lower than 100 xcexcm in order to obtain the specifications required for this fuel.
The aim of the present invention is a treatment process for a uranium dioxide powder obtained by dry method, to convert it into a powder suitable for direct use in the manufacture of MOX fuel.
According to the invention, the process for preparing a powder of sinterable uranium dioxide UO2 comprises the following stages:
1) to prepare an aqueous suspension of a powder of UO2 obtained by dry process from uranium hexafluoride, said suspension comprising 50 to 80% by weight of UO2 and at least one additive chosen among deflocculation agents, organic binders, hydrogen peroxide H2O2 and a powder of U3O8, in quantity such that the viscosity of the suspension does not exceed 250 mPa.sec, and
2) to atomise this suspension and dry it in a hot gas, at a temperature between 150xc2x0 and 300xc2x0 C., to obtain a powder of depleted UO2 with an average particle size of 20 to 100 xcexcm.
First of all, in this process, an aqueous suspension of uranium dioxide is prepared, comprising a very high dry matter content, but with a viscosity as low as possible, not exceeding 250 mPa.sec, so that it is suitable for the following operation of atomisation-drying of the suspension into calibrated granulates.
The fact that water is used to prepare the suspension is very interesting since it makes it possible to limit the quantities of organic products likely to introduce impurities into the final product to very low levels.
According to the invention, the method for preparing the suspension can be simple, quick, reproducible, and can lead to very fluid suspensions able to be carried by pumping to the injection nozzle of the atomiser without difficulty. Very high levels of dry material can be reached in order to obtain dense, full and completely spherical particles of powder. Furthermore, this process can be transposed to a production unit of industrial capacity.
In order to prepare this suspension, at least two additives are generally used, constituted respectively of:
1) at least one deflocculation agent, and
2) at least one additive chosen among organic binders, hydrogen peroxide and/or a powder of U3O8, these latter additives all playing the role of binder during the operation of atomisation-drying.
In the document EP-A-0 092 475 [7], the use of hydrogen peroxide is described for improving the resistance to compression of raw pellets of UO2 or UO2UO2 xe2x80x94by forming a layer of hydrated oxide on the initial powder. This layer is formed by pulverising a solution of H2O2 on the metal oxide powder, the quantity of H2O2 solution representing 2 to 15% of the weight of this powder. In this case, the quantity of H2O2 used is significant.
The deflocculation agent is intended to fluidify the suspension. It can be constituted by an organic product which can be easily eliminated, for example ammonium polymethacrylate such as the product marketed by the company Polyplastic S.A. under the name DARVAN C which is a 25% aqueous solution of ammonium polymethacrylate.
The quantity by weight of the deflocculation agent used (ammonium polymethacrylate) generally represents 0.03 to 0.16% by weight of dry matter in the suspension.
The organic binders are added to the suspension to encourage agglomeration of the powder during drying in an atomiser. Organic binders which can be eliminated easily are chosen. As examples of such binders, polyvinyl alcohol and polyethylene glycol can be mentioned.
Oxygenated water can play the same role as an organic binder, as can a powder of U3O8, but in the two cases, it can be advantageous to add a small proportion of an organic binder as well.
Thus, according to a first embodiment of the invention, the solution comprises a deflocculation agent and an organic binder, the quantity by weight of the organic binder representing from 0.3 to 1% of the weight of the dry matter in suspension.
According to a second embodiment of the invention, the suspension comprises a deflocculation agent and hydrogen peroxide H2O2, without organic binder, the quantity by weight of H2O2 representing from 0.2 to 0.4% of the weight of dry matter in the suspension. Oxygenated water can be added under the form of aqueous solution at 20% H2O2, for example.
According to a third embodiment of the invention, the suspension comprises a deflocculation agent, hydrogen peroxide and an organic binder such as polyvinyl alcohol, the quantity by weight of H2O2 representing 0.1 to 0.4% of the weight of dry matter of the suspension, and the quantity by weight of the organic binder representing 0.1 to 0.5% of the weight of the dry matter of the suspension.
In these three embodiments, the quantity by weight of the deflocculation agent such as ammonium polymethacrylate generally represents from 0.03 to 0.16 of the weight of dry matter in the suspension.
In the two embodiments where an organic binder is used, this can be polyvinyl alcohol or polyethylene glycol.
When the additive is constituted of a powder of U3O8, this can be obtained by controlled oxidation of the initial UO2 powder obtained by dry process. A quantity of U3O8 powder representing 10 to 20% of the weight of UO2 can be used, for example 15%.
In this case, the suspension can also comprise a deflocculation agent and an organic binder such as those used above in the proportions indicated above, for example 0.03 to 0.16% by weight of the deflocculation agent and 0.1 to 0.5% by weight of the organic binder relative to the weight of dry matter in the suspension, that is the total weight of UO2 and U3O8.
According to the invention, sintering admixtures under powder form can also be added to the powder suspension of UO2 and possibly U3O8, such as Cr2O3, TiO2, Al2O3 etc. and burnable poisons such as Gd2O3 or Er2O3. This makes it possible to obtain uniform dispersion of these admixtures and/or burnable poisons in the UO2 powder.
According to the invention, the dry matter of the suspension can thus be constituted either of UO2 powder alone, of the mixture UO2xe2x80x94U3O8, or of UO2-burnable poison mixtures and/or sintering admixture(s) and/or U3O8.
In order to prepare the suspension, one begins with a UO2 powder obtained by dry process, in the raw state, and one gradually adds water, the additives and possible admixtures to the UO2 powder and possibly U3O8, with mechanical or ultra-sound agitation, in one or several stages.
The following operation of atomisation-drying can be carried out in a standard atomisation-drying device, equipped for example with a turbine turning at high speed (centrifugal atomisation), with a nozzle fed under pressure (pressure or pneumatic atomisation) or an ultrasonic injection nozzle (sonotrode atomisation).
The setting of these different injection modes, either the diameter of the nozzle orifice, the rotation speed, the injection pressure or the ultrasonic frequency, must be carried out in such a way so as to produce the formation of a mist of micro-droplets in suspension, with for example an average diameter close to 50 xcexcm with a low droplet diameter dispersion, if possible from 20 to 100 xcexcm, no large-size droplet, for example of the order of a millimetre, being formed.
The drying atmosphere can be constituted of air or another gas exempt from oxygen, for example nitrogen or argon. The temperature of the gas is generally in the range between 150 and 300xc2x0 C., at least in the zone located just next to the injection nozzle. The drying can be carried out either with the current or against the current of the suspension feed or in mixed mode. The drying height of the droplets is preferably higher than 70 cm with a temperature at the bottom of the drying column higher than 100xc2x0 C. Preferably the atomisation-drying devices possess significant drying heights so as to obtain a final granulate with a minimum of residual humidity.
After this operation, a powder of UO2 is obtained, with the following properties:
a homogeneous particle size between 20 and 100xcexcm,
a granular cohesivity sufficient to withstand the different manipulations involved in preparing MOX fuels,
excellent flow properties, such as spontaneous flow for 200 xcexcm powder in a metallic flow meter provided with an orifice of diameter 15 mm,
a high apparent density, close to 2 gm/cm3,
an O/U ratio less than 2.15,
a low residual content of impurities,
good compaction capacity, and
excellent capacity for natural sintering giving, for example, after sintering, a density representing more than 97.5% of the theoretical density.
According to the invention, in certain cases one can use a supplementary thermal treatment of the powder obtained by atomisation-drying. This treatment can be carried out either at low temperature between 100 and 250xc2x0 C. to eliminate residual humidity from the powder, or at a higher temperature of 250 to 700xc2x0 C. to eliminate the totality of residual impurities from the powder, and if necessary set the O/U ratio by using an appropriate atmosphere for thermal treatment.
As far as the influence of thermal treatment is concerned, the morphology of the granulates changes very little during this treatment. In fact, they keep their sphericity and remain individualised, which makes it possible to preserve excellent flow properties.